1. Field of the Present Invention
This invention relates to magnetic separators particularly for fine particles. More specifically, a combination of forces are used to separate dry particles of different magnetic susceptibilities. The forces include magnetic forces along with suction, blowing, or a combination of suction and blowing. The magnetic force attracts and holds the ferromagnetic and strongly paramagnetic particles to the conveying surface while the suction and/or blowing lifts the diamagnetic and weakly paramagnetic particles off of the conveying surface. Thus a separation based on the magnetic properties of the particles is accomplished and agglomerating forces are overcome.
2. Description of the Prior Art
Fly ash from coal fired power plants has been used for a variety of purposes. One of the principal uses has been as a mineral admixture in Portland cement. A factor limiting the use of some fly ashes in Portland cement is that the Loss on Ignition (LOI) of the fly ash must be below 6.0% (ASTM C618-99). Some users require even lower concentrations of LOI. Fly ashes with LOI concentrations greater than those required by the user must be separated so that a fraction of the fly ash has an LOI concentration low enough to be used as an admixture in Portland cement.
Fly ash is generally composed of carbonaceous material—the source of LOI—mullite, quartz, ferrite spinel (magnetite), and hematite as discussed by McCarthy, G. and Thedchanamoorthy, A. “Semi-Quantitative X-Ray Diffraction Analysis of Fly Ash by the Reference Intensity Ratio Method,” Fly Ash and Coal Conversion By-Products: Characterization, Utilization and Disposal V. Materials Research Society Symposium Proceedings. 136, p. 70 (1989). Separated components of fly ash can be used for a variety of purposes. For instance, a low LOI fraction might be used for Portland cement, magnetite taken from the fly ash might be used for coal cleaning, or a low density fraction might be used in low density aggregate.
An inexpensive method for fly ash separation can create value-added products from fly ashes. The most common alternative to the beneficiation and use of fly ash is impoundment. Impoundment is an expense to power plants and can be a problem for plants with limited space. Problems of impoundment are avoided or limited by separating fly ash into value-added products, and the power plant gains a revenue source.
In the prior art, many separators exist to separate particles based on the physical and/or chemical properties of those particles. All separators depend on the existence of a physical or chemical difference between the types of particles to be separated. The more pronounced this difference is, the easier or better the separation becomes. By imposing a force (or forces) on the particles, the various types of particles will move in different directions.
In practical applications, however, the imposed force is never the only force experienced by the particles. Other forces common to nearly all particles are gravity, agglomeration due to moisture, agglomeration due to surface charges (for instance, charges acquired triboelectrically during handling), and air drag forces. Gravity is a force that is accepted, managed, and often used in separators. Air drag forces affect the fine particles more significantly than the coarse particles because of the larger surface area to volume ratio of fine particles. Likewise, surface charges result in electrostatic forces that generally affect fine particles more significantly than coarse particles. Electrostatic forces on individual particles are affected by the types of particles present, by the preparation of these particles, and by the transport of those particles. Therefore, electrostatic forces are difficult to control and use to separate a broad range of fine particles. Moisture readily collects on some particles causing them to agglomerate; however, in order for water to be a separation medium, the particles must be thoroughly wetted. If dry particles are to be recovered, the wetted particles must then be dried, adding complexity and cost to the separation process.
Magnetic susceptibility is one property of particles that can be used as a basis for separations. Particles with positive susceptibilities are paramagnetic or ferromagnetic and are attracted toward magnetic fields. Those with negative susceptibilities are diamagnetic and are repelled by magnetic fields. The magnitude of the magnetic force on paramagnetic and diamagnetic particles depends on the susceptibility of the particle, the mass of the particle, and the magnetic energy gradient which is the product of the magnetic field times its gradient. For ferromagnetic particles, the magnetic force is a product of the magnetic field gradient times the magnetic moment of the particle which is a function of the mass of the particle.
Many dry magnetic separators exist in the prior art. However, these separators are ineffective for fine particles—generally less than 10 microns as discussed by Gupta, R., Gidaspow, D., and Wasan, D. T., “Electrostatic Beneficiation of Eastern Oil Shales,” Chemical Engineering Communications. 108, pp. 50-51 (1991).
This ineffectiveness for fine particles is a result of the adhesive forces (i.e., moisture agglomeration forces and electrostatic forces) on the particles and the small mass of fine particles resulting in small magnetic forces. Gupta, et al., determined that the separation limit of their electrostatic fine particle separator was due to agglomeration of the particles (Gupta, et al., 62). Heavilon, et al., in U.S. Pat. No. 5,513,755 also identify the agglomeration of fly ash as the limiting factor hindering its separation.
In the present invention, two primary forces are applied to the particles in order to separate them. One is a magnetic force which is strongest for the most paramagnetic and ferromagnetic particles. In addition to the magnetic force, suction, blowing, or a combination of suction and blowing is applied to the particles. Suction and blowing create gas (typically air) drag forces that are common to all particles but have the greatest effects on the finest particles. Uncontrolled forces such as electrostatic forces and agglomeration due to moisture remain; however, we have discovered that the forces of agglomeration can be overcome by using strong magnetic forces along with suction and/or blowing. Also, by reprocessing one or both of the separated fractions, additional fractions with varying magnetic susceptibilities can be removed. An example will be shown in which fly ash of 3900 μemu/(g·Oe) susceptibility was separated into high and low susceptibility fractions, and the low susceptibility fraction was again separated with a stronger magnetic field and magnetic field gradient. This process was repeated until a sample of 140 μemu/(g·Oe) was removed.
By increasing the velocity of the gas (typically air), the suction and/or blowing can dominate all other forces. By reducing the velocity, they can dominate all but the strongest remaining forces. For strongly paramagnetic and ferromagnetic particles, the magnetic force can be made to be the strongest force on these particles and, therefore, is not overcome by suction and/or blowing. In this manner, the diamagnetic and weakly paramagnetic particles are removed from the conveying surface by the gas drag and are conveyed away by the gas stream. The strongly paramagnetic and ferromagnetic particles are mechanically conveyed away from the gas drag and the magnetic mechanism where they then fall off of the conveying surface into a collection vessel. This leaves the conveying surface clean.
Separations have been successfully accomplished using gas velocities near the conveying surface of less than 10 m/s to over 140 m/s and using magnetic field strengths below 6000 gauss to more than 10,000 gauss. Particles have been separated with susceptibilities of more than 30,000 μemu/g·Oe to less than 150 μemu/g·Oe and with sizes of more than 300 microns to less than 10 microns.